I'm using Google index checker and would like to know what it actually checks for.
I tested with a list of 35 of my websites, in all instances the home page and with correct http or https. and www or not www,
I only get 12 indexed, and while some are indeed not indexed, at least 80% are in reality when checking by hand on Google.

What's the search string that's used?

PS I get 18 indexed on Yahoo, and 14 on Bing, and I never tried to index on either one of them.